This study proposes to investigate central nervous system control strategies used to produce accurate movements. Control of voluntary movement is viewed in this study as a stratified process with several levels of control: 1) movements are initiated by high level construction of open-loop motor commands, which are 2) executed form a reference frame stabilized by a postural control system, and are 3) modified by sensory feedback during both command execution ("concurrent feedback") and after movement completion using knowledge of results ("delayed feedback"). Skilled movements and isometric force production are evaluated in terms of these three control mechanisms. Adult, human subjects will track various waveforms presented on a visual display by exerting force on a manipulandum with their elbow musculature. Comparison will be made of motor accuracy under a variety of conditions affecting: 1) the predictability of the stimulus (tracking waveform) amplitude, 2) the availability of visual feedback, 3) the degree of postural stability, and 4) movement of the elbow joint. In most experiments subjects will track step waveforms on an oscilloscope screen while, in one experiment, designed to distinguish between strategies subserving timing and metrical movement precision, graphical displays will be more complex. Electromyographic activity will be recorded from appropriate muscles during tracking experiments in order to characterize movement strategies at a level of peripheral neural commands to muscles. Amputees fitted with myoelectrically controlled arm prostheses will be used in several phases of this study. In one experiment, these individuals will be used as a model of the "deafferented arm" in order to characterize this role of pehipheral somesthetic feedback in the control of accurate movements in normal subjects. In addition, a pilot experiment is proposed which directly addresses the problem of user control of multiple degree-of-freedom powered arms. It is hoped that the latter experiment will lead to the development of a large-scale research and training program for amputees at Good Samaritan Hospital and Medical Center.